1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatizations reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. The increased sensitivity of these selective catalysts to sulfur in the feed also is known. Nevertheless, this dehydrocyclization technology has not been commercialized during the intense and lengthy development period. The extreme catalyst sulfur intolerance of current reforming catalysts selective for dehydrocyclization, providing surprising results when sulfur is precluded from the feed according to the process of the present invention, is only now being recognized.
3. Related Art
U.S. Pat. No. 2,618,586 (Hendel) discloses a process for removing relatively small amounts of sulfur-containing compounds from a petroleum liquid using an adsorbent which could be manganese oxide. U.S. Pat. No. 3,063,936 (Pearce et al.) discloses a desulfurization process combining sulfuric acid treatment, contact with a material which may be manganese oxide and contact with a hydrodesulfurization catalyst. However, neither Hendel nor Pearce et al. suggest the catalytic reforming process of the present invention.
U.S. Pat. No. 3,898,153 (Louder et al.) teaches a catalytic reforming process including chloride removal, hydrodesulfurization, and zinc oxide adsorbent to reduce the sulfur content of the reformer feed to as low as 0.2 ppm. U.S. Pat. No. 4,634,515 (Bailey et al.) discloses a nickel-catalyst sulfur trap downstream of a hydrofiner to reduce sulfur content to preferably below 0.1 ppm before a reforming unit. However, neither Louder et al. nor Bailey et al. contemplate the first reforming zone and manganese component precluding sulfur from the feed to a second reforming zone of the present invention.
U.S. Pat. Nos. 4,225,417 and 4,329,220 (Nelson) teach a reforming process in which sulfur is removed from a reforming feedstock using a manganese-containing composition, particularly directed to sulfur removal in a halogen atmosphere. Preferably, the feed is hydrotreated and the sulfur content is reduced by the invention to below 0.1 ppm. U.S. Pat. Nos. 4,534,943 and 4,575,415 (Novak et al.) teach an apparatus and method, respectively, for removing residual sulfur from reformer feed using parallel absorbers for continuous operation; ideally, sulfur is removed to a level of below 0.1 ppm. Neither Nelson nor Novak et al., however, suggest the two reforming zones and resulting preclusion of feed sulfur to the second reforming zone of the present invention.
U.S. Pat. No. 4,456,527 (Buss et al.) discloses the reforming of a hydrocarbon feed having a sulfur content of as low as 50 ppb (parts per billion) with a catalyst comprising a large-pore zeolite and Group VIII metal. A broad range of sulfur-removal options are disclosed to reduce the sulfur content of the hydrocarbon feed to below 500 ppb. U.S. Pat. No. 4,741,819 (Robinson et al.) is drawn to a method for removing residual sulfur from a hydrotreated naphtha feedstock comprising contacting the feedstock with a less-sulfur sensitive reforming catalyst, a sulfur sorbent, and a highly selective reforming catalyst. Both Buss et al. and Robinson et al. teach that the sensitive reforming catalyst preferably contains an alkaline earth metal, in contrast to the alkali metal of the present catalyst. Further, the broad list of sulfur-removal options and metals of Buss et al and Robinson et al. do not include a manganese sulfur sorbent and preferably lead one to select a zinc oxide sorbent. In summary, neither Buss et al. nor Robinson et al. teach the reforming process combination of the present invention using a manganese component to preclude sulfur as elucidated hereinafter from the feed to a second reforming zone.
U.S. Pat. No. 4,831,206 (Zarchy) discloses a hydrocarbon conversion process comprising sulfur conversion, liquid-phase H.sub.2 S removal with zeolite, and vaporization of the product to the reaction zone. Zarchy requires condensation and vaporization of the hydrocarbon stream, however, and does not teach the use of a manganese component to achieve the substantially sulfur-free effluent of the present invention. Pending application Ser. No. 354,653 (Zarchy) discloses an isomerization process comprising a first conversion zone, a hydrogen sulfide recovery zone and a second conversion zone, but does not suggest the present reforming process.